CPS 356: Introduction to Computer
Networks
Lecture 2: Network Architectures
Reference: Chapter 1 of [PD]
Xiaowei Yang
xwy@cs.duke.edu
Overview
• Updated course administrative stuff
– Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of how the Internet works
Updated Grading Policy
• Old
– Class participation and pop quizzes: 20%
– Assignments: 50%
• In a group assignment, both students get the same grade for
the assignment
– Exams: 30%
• New
– Class participation and pop quizzes: 10%
– Assignments: 60%
• In a group assignment, both students get the same grade for
the assignment
– Exams: 30%
Office hours
• Instructor
– Fridays: 3-5pm
• TA
– Tuesdays 7-9pm
Discussion Forum
• Piazza sign up link
– piazza.com/duke/spring2014/compsci356
Overview
• Updated course administrative stuff
– Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of how the Internet works
1st Mission of this course
• Understand the concepts and design principles that make
the Internet work
• Design paradigm
– Identify requirements, brainstorm design choices/mechanisms,
make design decisions
– What requirements make sense to you?
•
•
•
•
•
Scalable connectivity
Cost-effective resource sharing
Support for different types of services
Manageability
…
– It remains an open challenge how to incorporate other
requirements such as security into the Internet design
Features of computer networks
• Generality
• Carry many different types of data
• Support an unlimited range of applications
What’s the Internet?
• The Internet is a large-scale general-purpose
computer network.
– Run more than one applications
• The Internet transfers data between computers.
• The Internet is a network of networks.
Design requirements and
techniques to meet them
1.
2.
3.
4.
Scalable connectivity
Cost-effective resource sharing
Support for common services
Manageability
1. Scalable Connectivity
• A network must provide connectivity among a set of
computers
– Open vs close: to connect all computers or a subset of them?
– Internet is an open network
• Scalability: A system is designed to grow to an arbitrary
large size is said to scale
– How to connect an arbitrary large number of computers on a
network?
Connectivity recursively occurs at different
levels
Point-to-Point
Multiple-Access
• Link-level: connect two or more computers via a physical
medium
• Computers are referred to as nodes
• The physical medium is referred to as a link
Switching
• Switching is a mechanism to achieve connectivity
• Nodes that are attached to at least two links forward data from one
link to another link
• They are called switches
• Computers outside the cloud are called hosts
• A question: switch vs router, what can become a switch?
• Circuit switching
– Sets up a circuit before nodes can communicate
– Switches connect circuits on different links
• Packet switching
– Data are split into blocks of data called packets
– Store and forward
– Nodes send packets and switches forward them
Internetworking:
Another way to achieve
connectivity
• An internetwork of networks
– Each cloud is a network/a multiple-access link
– A node that is connected to two or more networks is commonly called a router
• Speaks different protocols than switches
– An internet can be viewed as a “cloud.” We can recursively build larger clouds
by connecting smaller ones
– Autonomous system (AS)
Addressing
and routing
•
•
•
•
Physical connectivity != connectivity
Addressing and routing are mechanisms to achieve connectivity
Nodes are assigned addresses
Routers compute how to reach them by running routing protocols
– intra-AS: OSPF, RIP, IS-IS
– Inter-AS: BGP
2. Cost-effective resource sharing
• Question: how do all the hosts share the network when
they want to communicate with each other?
– Use at the same time
– Fair
• Multiplexing: a system resource is shared among multiple
users
– Analogy: CPU sharing
• Mechanisms to multiplexing
– Time-division multiplexing (TDM)
– Frequency-division multiplexing (FDM)
– Statistical multiplexing
Multiplex
Demultiplex
TDM and FDM
Example:
TDM
4 users
frequency
time
FDM
frequency
time
Problems with FDM and TDM
• What if a user does not have data to send all
the time?
– Consider web browsing
–  Inefficient use of resources
• Max # of flows is fixed and known ahead of
time
– Not practical to change the size of quantum or add
additional quanta for TDM
– Nor add more frequencies in FDM
Statistical Multiplexing
10 Mb/s
Ethernet
A
B
statistical multiplexing
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
• The physical link is shared over time (like TDM)
• But does not have fixed pattern  statistical
multiplexing
– Sequence of A & B packets are sent on demand, not
predetermined slots
Pros and Cons
• Assumption: traffic is largely bursty
• Pros: Resources are not wasted when hosts are idle
• Cons: No guarantee flows would have their turns to
transmit
• Some possible fixes:
– Limit maximum packet size
– Scheduling which packets got transmitted, e.g., fair
queuing
Maximum Packet Size
• Divide an application message into blocks of
data  packets
– Segments, frames
• Maximum packet size limit
– Flows send on demand
– Must give each flow its turn to send
– Solution: defines an upper bound on the size of the
block of data
Packet scheduling
• Scheduling: which packet to send
• First come first serve (FIFQ)
• Weighted fair queuing
Switching vs multiplexing
• TDM and FDM are used in circuit switching
– Require a setup as max # of flows is fixed
• SM is used in packet switching
Congestion
• Aggregate incoming rate > outgoing rate
• An open question
• A large buffer can help temporary congestion
Packet switching versus circuit switching
Packet switching allows more users to use network!
• 1 Mb/s link
• each user:
– 100 kb/s when “active”
– active 10% of time
• circuit-switching: fixed capacity
– 10 users
• packet switching:
– with 35 users, probability >
10 active less than .0004
N users
1 Mbps link
3. Support for common services
• Application developers want a network to provide
services that make application programs communicate
with each other, not just sending packets
– E.g. reliably delivering an email message from a sender to a
receiver
• Many complicated things need to happen
– Can you name a few?
• Design choices
– Application developers build all functions they need
– Network provides common services  a layered network
architecture
• Build it once, and shared many times
• Interactive
request/reply
• Streaming of data
• Bulk data transfer
• …
• Key challenges: what services/channels to provide that can satisfy
most applications at lowest costs?
• Approach: identify common patterns, then decide
– What functions to implement
– Where to implement those functions
• We will discuss end-to-end arguments in future class
Ex: how to provide reliability as a common service
• Failures may occur at different scopes
– Bit transmission errors
– Packet loss
– Component failures: link, node
• Design choices
– Link layer
– Every hop in the router
– End systems
• In future classes, we will discuss how to cope with these
failures
4. Manageability
• Manage the network as it grows and when
things go wrong
• An open research challenge
– Datacenter networks
– Backbones
– Home networks
• IP cameras, printers, network attached storage
Overview
• Updated course administrative stuff
– Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of how the Internet works
Network Architectures
• Many ways to build a network
• Use network architectures to characterize
different ways of building a network
• The general blueprints that guide the design
and implementation of networks are referred to
as network architectures
Central concepts
• Layering
• Protocols
Layering
Not so strict
• An abstraction to handle complexity
– A unifying model that capture important aspect of a
system
– Encapsulate the model in an object that has an
interface for others to interact with
– Hide the details from the users of the object
Advantages of layering
• Simplify the design tasks
– Each layer implements simpler functions
• Modular design
– Can provide new services by modifying one layer
Protocols
• The abstract objects that make up the layers of a network
system are called protocols
• Each protocol defines two different interfaces
– Service interface
– Peer interface
A protocol graph
• Peer-to-peer communication is indirect
– Except at the hardware level
• Potentially multiple protocols at each level
• Show the suite of protocols that make up a
network system with a protocol graph
A sample protocol graph
Protocol standardization
• Standard bodies such as IETF govern procedures
for introducing, validating, and approving
protocols
– The Internet protocol suite uses open standard
• Set of rules governing the form and content of a
protocol graph are called a network architecture
We reject kings, presidents, and
voting. We believe in rough
consensus and running code
- David Clark
Encapsulation
• Upper layer sends a message using the service
interface
• A header, a small data structure, to add information
for peer-to-peer communication, is attached to the
front message
– Sometimes a trailer is added to the end
• Message is called payload or data
• This process is called encapsulation
Multiplexing & Demultiplexing
• Same ideas apply up and down the protocol graph
Examples of Network
Architectures
The protocol graph of Internet
Applicatoin layer
Transport layer
Network layer
Link layer
• No strict layering. One can do cross-layer design
• Hourglass shaped: IP defines a common method for exchanging packets
among different networks
• To propose a new protocol, one must produce both a spec and one/two
implementations
Functions of the Layers
• Link Layer:
– Service:
•
– Functions:
Reliable transfer of frames over a link
Media Access Control on a LAN
Framing, media access control, error checking
Network Layer:
– Service:
– Functions:
Move packets from source host to destination host
Routing, addressing
• Transport Layer:
– Service:
Delivery of data between hosts
– Functions:
Connection establishment/termination, error control,
flow control
• Application Layer:
– Service:
Application specific (delivery of email, retrieval of
HTML
documents, reliable transfer of file)
– Functions:
Application specific
The Open Systems Interconnection (OSI)
architecture
Seven-layer
• International Telecommunications Union (ITU) publishes
protocol specs based on the OSI reference model
– X dot series
• Physical layer: handles raw bits
• Data link layer: aggregate bits to frames. Network adaptors
implement it
• Network layer: handles host-to-host packet delivery. Data
units are called packets
• Transport: implements process channel. Data units are called
messages
• Session layer: handles multiple transport streams belong to the
same applications
• Presentation layer: data format, e.g., integer format, ASCII
string or not
• Application layer: application specific protocols
Summary of New Terms
• Layering is an abstraction that captures important aspects of the
system, provides service interfaces, and hides implementation details
• Protocols are abstract objects that make up the layers of a network
system are
• A protocol graph represents protocols that make up a system
– Nodes are protocols
– Links are depend-on relations
• Set of rules governing the form and content of a protocol graph are
called a network architecture
• Attaching a header/trailer to an upper layer data unit is referred to as
encapsulation
An Example
A simple TCP/IP Example
argon.tcpip-lab.edu
("Argon")
neon.tcpip-lab.edu
("Neon")
Web request
Web page
Web client
Web server
• A user on host argon.tcpip-lab.edu (“Argon”) makes web
access to URL
http://neon. tcpip-lab.edu/index.html.
• What actually happens in the network?
HTTP Request and HTTP response
Argon
HTTP client
Neon
HTTP request
HTTP server
HTTP response
• Web server runs an HTTP server program
• HTTP client Web browser runs an HTTP client
program
• sends an HTTP request to HTTP server
• HTTP server responds with HTTP response
HTTP Request
GET /example.html HTTP/1.1
Accept: image/gif, */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0
Host: 192.168.123.144
Connection: Keep-Alive
HTTP Response
HTTP/1.1 200 OK
Date: Sat, 25 May 2002 21:10:32 GMT
Server: Apache/1.3.19 (Unix)
Last-Modified: Sat, 25 May 2002 20:51:33 GMT
ETag: "56497-51-3ceff955"
Accept-Ranges: bytes
Content-Length: 81
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
<HTML>
<BODY>
<H1>Internet Lab</H1>
Click <a href="http://www.tcpip-lab.net/index.html">here</a> for the Internet Lab
webpage.
</BODY>
</HTML>
• How does the HTTP request get from Argon to Neon ?
From HTTP to TCP
Argon
Neon
HTTP client
HTTP request / HTTP response
HTTP server
TCP client
TCP connection
TCP server
• To send request, HTTP client program
establishes an TCP connection to the HTTP
server Neon.
• The HTTP server at Neon has a TCP server
running
Resolving hostnames and port
numbers
• Since TCP does not work with hostnames and
also would not know how to find the HTTP
server program at Neon, two things must happen:
1. The name “neon.tcpip-lab.edu” must be
translated into a 32-bit IP address.
2. The HTTP server at Neon must be identified
by a 16-bit port number.
Translating a hostname into an IP
address
neon.tcpip-lab.edu
HTTP client
128.143.71.21
argon.tcpip-lab.edu
DNS Server
128.143.136.15
• The translation of the hostname neon.tcpip-lab.edu into an IP
address is done via a database lookup
– gethostbyname(host)
• The distributed database used is called the Domain Name
System (DNS)
• All machines on the Internet have an IP address:
argon.tcpip-lab.edu
128.143.137.144
neon.tcpip-lab.edu
128.143.71.21
Finding the port number
• Note: Most services on the Internet are reachable via well-known
ports. E.g. All HTTP servers on the Internet can be reached at
port number “80”.
• So: Argon simply knows the port number of the HTTP server at a
remote machine.
• On most Unix systems, the well-known ports are listed in a file
with name /etc/services. The well-known port numbers of some of
the most popular services are:
ftp
21
finger 79
telnet
23
http
80
smtp
25
nntp 119
Requesting a TCP Connection
argon.tcpip-lab.edu
connect(s, (struct sockaddr*)&sin, sizeof(sin))
HTTP client
Establish a TCP connection
to port 80 of 128.143.71.21
TCP client
• The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish
a connection to port 80 of the machine with address 128.141.71.21
Invoking the IP Protocol
argon.tcpip-lab.edu
TCP client
Send an IP datagram to
128.143.71.21
IP
• The TCP client at Argon sends a request to establish a connection to port 80 at
Neon
• This is done by asking its local IP module to send an IP datagram to
128.143.71.21
• (The data portion of the IP datagram contains the request to open a
connection)
Sending the IP datagram to the
default router
• Argon sends the IP datagram to its default router
• The default gateway is an IP router
• The default gateway for Argon is
Router137.tcpip-lab.edu (128.143.137.1).
Invoking the device driver
argon.tcpip-lab.edu
IP module
Send an Ethernet frame
to 00:e0:f9:23:a8:20
Ethernet
• The IP module at Argon, tells its Ethernet device driver to send an
Ethernet frame to address 00:e0:f9:23:a8:20
• Ethernet address of the default router is found out via ARP
The route from Argon to Neon
• Note that the router has a different name for each of its interfaces.
Sending an Ethernet frame
• The Ethernet device driver of Argon sends the
Ethernet frame to the Ethernet network interface
card (NIC)
• The NIC sends the frame onto the wire
Forwarding the IP datagram
•
The IP router receives the Ethernet frame at interface 128.143.137.1
1. recovers the IP datagram
2. determines that the IP datagram should be forwarded to the interface
with name 128.143.71.1
•
The IP router determines that it can deliver the IP datagram directly
Invoking the Device Driver at the
Router
router71.tcpip-lab.edu
IP module
Send a frame to
00:20:af:03:98:28
Ethernet
• The IP protocol at Router71, tells its Ethernet device
driver to send an Ethernet frame to address
00:20:af:03:98:28
Sending another Ethernet frame
• The Ethernet device driver of Router71 sends
the Ethernet frame to the Ethernet NIC, which
transmits the frame onto the wire.
Data has arrived at Neon
• Neon receives the Ethernet frame
• The payload of the Ethernet frame is an
IP datagram which is passed to the IP
protocol.
• The payload of the IP datagram is a TCP
segment, which is passed to the TCP
server
neon.tcpip-lab.edu
HTTP server
TCP server
IP module
Ethernet
Summary
• Updated course administrative stuff
– Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of
• how the Internet works